1. Field of Invention
This invention relates to high voltage solar cells, apparatus for use in forming a high voltage solar cell, a process for forming the apparatus and a process for using the apparatus.
2. Description of Related Art
It is well-known that under light illumination photovoltaic (PV) solar cells generate direct electric current (DC) at a certain voltage. Current solar cells produced by standard photovoltaic cell manufacturing processes that use crystalline silicon semiconductor material usually generate an electrical short circuit current density (Jsc) of about 32-36 mA/cm2 and an open circuit voltage (Voc) of about 600-620 mV under standard illumination of 1000 W/m2. In order to achieve a higher voltage output several photovoltaic (PV) cells must be interconnected in series to create a PV module.
Currently, most PV modules employ square or semi-square PV cells measuring about 4 to about 6 inches on a side. These cells generate a short circuit current (Isc) of up to about 3.5 A for the 4-inch cell to about 9A for the 6-inch cell at an open circuit voltage (Voc) of about 600 mV to about 620 mV, respectively. Recently introduced 8-inch PV cells generate even higher short circuit currents (Isc) up to about 15 A. These larger 8-inch cells have several advantages. One advantage is that production costs measured in dollars per watt of generated power are lower than with conventional smaller sized cells. In addition these larger cells have a greater potential efficiency due to the lower ratio of edge length to area.
In spite of these advantages PV module manufacturers are still reluctant to use 8-inch cells in module production because 8-inch cells generate very high electric current at low voltage and this requires provisions for very low resistance current collection on the front side of the cell in order to minimize voltage drops. This problem can be solved by using more current collecting bus-bars such as 3 bus-bars instead of the conventional 2 bus-bars. However the use of 3-bus-bars requires new tooling and equipment which increases manufacturing costs of these cells. In addition, the size of standard PV modules is limited by manufacturing processes and therefore the larger the area of cells used in a PV module, the smaller the number of cells that fit into the module, which limits the output voltage of the module, even if all cells are interconnected in series. As there is a growing need to convert DC power into AC power using inverters, DC input voltages must be on the order of 300V in order to achieve conversion efficiencies that are cost effective. Voltages in this range may be achieved only if at least 600 conventional PV cells are interconnected in series. Therefore the larger the size of PV cell incorporated into a PV module, the lower the number of cells and therefore the lower the voltage produced by the module relative to PV modules of the same area that employ PV cells with smaller areas.
Conventional approaches to solve this problem involve dividing PV cells in such a way that mechanical integrity is preserved while the cell performs electrically as though it were more than one cell. There are various ways of achieving this.
U.S. Pat. No. 5,164,019 to Sinton entitled “Monolithic Series-Connected Solar Cells having Improved Cell Insulation and Process of Making the Same” describes an array of series-connected PV sub-cells that are built in a monolithic semiconductor substrate and electrically insulated from each other by grooves in the cell surface. The grooves that separate sub-cells are produced either on a front or on a back side of the semiconductor substrate and the depth of the groove is controlled to create a crack inside the semiconductor bulk material to completely break the semiconductor substrate between the sub-cells. Mechanical integrity is provided by metallization that interconnects the PV sub-cells. Since this technology requires a complete break in the substrate the final product is very fragile. This technology is quite complicated and expensive, and may not be cost effective for large-scale production of PV cells and PV modules.
U.S. Pat. No. 4,933,021 to Swanson entitled “Monolithic Series-Connected Solar Cells Employing Shorted p/n Junctions for Electrical Insulation” describes the use of electrical insulation between PV sub-cells on a single substrate by forming a plurality of p/n junctions in the substrate between adjacent sub-cells and shorting the p/n junctions by metallization serially interconnecting adjacent sub-cells. Again, this technology is quite expensive and probably not cost effective for large-size PV devices.
U.S. Pat. No. 4,376,872 to Evans, et al. entitled “High voltage V-groove solar cell” describes a high voltage multifunction solar cell comprising a plurality of discrete voltage generating regions or unit cells which are formed in a single semiconductor wafer and which are connected together so that the voltages of the individual cells are additive. The unit cells comprise doped regions of opposite conductivity types separated by a gap. V-shaped grooves are formed in the wafer and configured so that ions of one conductivity type can travel in one face of a groove while the other face is shielded. The V-shaped grooves function to interconnect the unit cells in series rather than to separate the unit cells. This process is complex and probably not cost effective for mass production of photovoltaic cells.
U.S. Pat. No. 4,278,473 to Borden entitled “Monolithic series-connected solar cell” describes monolithic series-connected solar sub-cells that are defined as separate sub-cells by electrochemically produced grooves that penetrate from a top surface into the semiconductor substrate to an insulating substrate. The grooves have walls on which interconnections between sub-cells are formed by providing a connection from a top part of a cell to a contact ledge formed in a base region of an adjoining cell. This technology is complicated, expensive and likely applicable only for small electronic and photovoltaic devices.
U.S. Pat. No. 4,173,496 to Chiang, et al. entitled “Integrated Solar Cell Array” describes an integrated, monolithic array of solar cells wherein isolation between cells permits series interconnection of the cell to provide an output voltage for the array equal to the sum of the voltage of the unit cells. Isolation is provided between neighboring cells by a pattern of grooves having walls that are subsequently coated with metallization, an oxide layer and selective doping to create P+ and N+ regions to provide an electrical connection between cells and to eliminate spurious shunt current between them. This technology is complicated, expensive and likely applicable only for an integrated monolithic array of solar cells and potentially inefficient for large PV cells and modules.
U.S. Pat. No. 4,603,470 to Yamazaki entitled “Method of Making Plurality of Series Connected Solar Cells Using Multiple Groove Forming Processes” describes a method for interconnecting a plurality of non-single-crystal semiconductor solar cells by forming a plurality of grooves in a metallization layer of a substrate. The grooves do not appear to penetrate into the bulk of the substrate. This technology cannot be applied on crystalline silicon semiconductors and therefore is unlikely to be applicable to mass produced PV cells and modules.
U.S. Pat. No. 4,517,403 to Morel, et al. entitled “Series Connected Solar Cells and Process of Formation” describes a photovoltaic device that has a continuous thin film with a plurality of spaced photovoltaic regions thereon and front and back electrode portions associated with each of the photovoltaic regions. Electrical connection between the regions is provided directly through the film itself, from each back electrode portion to the front electrode portion of an adjacent region. Thus, at least two of the photovoltaic regions are connected in series to increase the output voltage of the device. This technology is applicable to thin film semiconductor material and would probably not be used for mass production of PV cells and modules.
An article entitled “Monolithically Series-Connected Crystalline Si Wafer Cells for Portable Electronic Devices” (Adam Hammud, Barbara Terheiden, Richard Auer and Rolf Brendel: 31st IEEE Photovoltaic Specialists Conference 2005, IEEE Catalog No: 05CH37608C, ISBN: 0-7803-8708-5) describes a simple process for the fabrication of solar mini-modules from crystalline Si wafers. This process involves p/n junction formation, passivation by plasma enhanced chemical vapor deposition, selective plasma etching, electrical interconnection between a semiconductor emitter and base by aluminum evaporation, Si-wafer fixation on glass substrate and creation of separated solar PV sub-cells by dicing and subsequent plasma etching. The best out of thirty PV modules comprised of 6 series-connected PV sub-cells is described to provide an efficiency of 11% and an open circuit voltage (Voc) of 3.624V. This technology appears to relate to separation of complete sub-cells by dicing and affixing the individual sub-cells onto the glass substrate. Although the authors claimed that the technology is simple it may be too complicated and expensive to satisfy PV industry requirements.
U.S. Pat. No. 4,330,680 to Goetzberger entitled “Integrated series-connected solar cell” describes a row of strip-shaped semiconductor junctions arranged on each of two surfaces of a semiconductor substrate possessing a high ohmic resistance. The junctions alternate in having p+ and n+ conduction characteristics and are parallel to each other and spaced apart at intervals in such a way that a semiconductor junction having a p+-conduction characteristic on one surface of the semiconductor substrate is located opposite a semiconductor junction having an n+ conduction characteristic on the other surface, for example. Printed circuit tracks are arranged on the substrate to connect, in each case, one set of solar cell junctions with a neighboring set, in series connection. Essentially, this technology provides a way to interconnect solar cells in series by means of proper fabrication of p+ and n+ conductive regions on the semiconductor substrate. This technology is strongly dependent on complicated and expensive microelectronic equipment and is unlikely to be cost-effective to satisfy PV industry requirements.
U.S. Pat. No. 6,441,297 to Keller, et al entitled “Solar Cell Arrangement” describes a solar cell arrangement comprising series-connected solar PV sub-cells. A semiconductor wafer acts as a common base material for a plurality of solar PV sub-cells. Recesses are provided in the wafer for delimiting individual, series-connected solar PV sub-cells. Some of the recesses extend from the top surface of the semiconductor wafer, through the wafer itself to the bottom surface and some bridge segments are left to continue the recesses as far as the wafer edge, to mechanically interconnect the sub-cells. This technology requires dicing which weakens the semiconductor wafer making the final product fragile and requiring mounting on a solid substrate.
Generally, the above references employ mechanical and/or microelectronic process of treating a common semiconductor substrate, to create PV sub-cells interconnected in-series on the semiconductor substrate itself, however, generally each reference describes sophisticated and expensive technologies that are unlikely to be practical for large scale PV module fabrication using large wafers.